diff --git a/tools/python/requirements.txt b/tools/python/requirements.txt
index ddbaef3..81d306d 100644
--- a/tools/python/requirements.txt
+++ b/tools/python/requirements.txt
@@ -15,6 +15,7 @@
 validators
 yapf
 sympy
+pyyaml
 
 # TODO(phil): Migrate to absl-py. These are abandoned as far as I can tell.
 python-gflags